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\f0\b\fs24 \cf0 Across the U.S., Heaviest Downpours on the Rise
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by Climate Central\
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\cf0 May 27, 2015 \'96 Record-breaking rain across Texas and Oklahoma this week caused widespread flooding, the likes of which the region has rarely, if ever, seen. For 7 locations there, May 2015 has seen the most rain of any month ever recorded, with 5 days to go and the rain still coming. While rainfall in the region is consistent with the emerging El Ni\'f1o, the unprecedented amounts suggest a possible climate change signal, where a warming atmosphere becomes more saturated with water vapor and capable of previously unimagined downpours.\
Several people have been killed and hundreds have been rescued from their homes. Texas Gov. Greg Abbott has already declared disaster areas in 37 counties. These torrential downpours follow weeks of unusually rainy weather across the Southern Plains. They stack up to a broader trend in the region, and across the U.S., toward more heavy precipitation.\
Across most of the country, the heaviest downpours are happening more frequently, delivering a deluge in place of what would have been routine heavy rain. Climate Central\'92s new analysis of 65 years of rainfall records, at thousands of stations nationwide, found that 40 of the lower 48 states have seen an overall increase in heavy downpours since 1950. The biggest increases are in the Northeast and Midwest. In the past decade, they have seen 31 and 16% more heavy downpours than in the 1950s.\
These intense bouts of rain can wreak havoc on communities. They cause flooding; close schools, businesses, highways and airports; compromise roads and bridges; trigger sewage overflows; routinely produce million of dollars in damage; and kill people.\
In Nashville, for example, a 2010 record downpour dumped 13.6 inches of rain in just 2 days, causing an estimated $2 billion in damage. 11 people were killed, 11,000 homes and businesses were damaged, and 2,700 businesses suffered closures. Similarly, intense rain in Detroit in August 2014 killed 2, caused an estimated $1.1 billion in damage, and affected 118,000 homes and businesses.\
Heavy rain events may also pose a potential health risk. One recent study found that about 1/2 of all waterborne disease outbreaks in the U.S. (between 1948 and 1994) were linked to days of very heavy rain.\
Extreme heavy downpours are consistent with what climate scientists expect in a warming world. With hotter temperatures, more water evaporates off the oceans, and the atmosphere can hold more moisture. Research shows that the amount of water vapor in the atmosphere has already increased.\
That means that \'a0there is often a lot more water available to come down as rain. Climate scientists have already shown that increasing greenhouse gas concentrations, as a result of human activity, are partially responsible for the average global increase in heavy precipitation.\
Our analysis examines the heaviest downpours \'97 the days where total precipitation exceeded the top 1% of all rain and snow days \'97 at over 3,000 rain gauges distributed across the country, over the period 1950-2014. Although some cities have rain gauges that have been around since the 1860s, by the 1950s, about 90% of the current list of 3,000 stations were in existence, giving us a consistent 65-year period of study over the whole U.S.. The vast majority of this heavy precipitation came as rain, although in a few rare instances, major snowfalls also count toward these large events.\
Heavy precipitation is highly localized, far more than extreme heat, which typically covers relatively large areas. On a 98\'b0F day in New York City, similar high temperatures are usually also found across the entire metro area. On the other hand, an extreme rainfall event can inundate the Long Island suburbs and leave the city itself relatively unscathed.\
This makes heavy rainfall trends difficult to measure, because rain gauges are not always located where extreme rains occur. Moreover, local heavy precipitation trends may not accurately reflect changing patterns happening at a larger, regional level. As a consequence, even in regions or states where there is a strong increasing trend in heavy precipitation, the trend at an individual precipitation gauge that represents the official total for a city may be equivocal, flat, or even down.\
Our state level analyses of extreme precipitation events shows a strong increasing trend since the 1950s. 40 of 48 states show at least some increasing incidence. Consistent with earlier research, 6 of the top 10 states with the biggest increases in number of days with heavy downpours are in the Northeast; they include Rhode Island, Maine, and New Hampshire, which have seen the number of heavy rain events in the last decade increase by at least 50%, compared to the 1950s.\
Our analysis of cities and metro areas, on the other hand, reveals the highly localized and random nature of extreme rain events, and the difficulty of detecting these events, even with 3,000 rain gauges across the country. In a number of states and regions with clear increasing trends, individual locations show a weak trend or no clear trend at all. This apparent inconsistency says little about the overall trend in the heaviest precipitation events, but a lot about the weaknesses of single-point measurements for detecting trends in extreme precipitation.\
An example of this phenomenon is Boston, where the local trend is flat, but at the state level, Massachusetts has seen a relatively steady increase in heavy downpours since 1950.\
Of course, many individual locations show strong increasing trends in the heaviest precipitation events. The top 50 cities with the strongest increases [table in original] start with McAllen TX, Portland ME, Philadelphia, New York City, and Louisvile KY. The 2nd 5 are Visalia CA, Harrisburg PA, Houston, Augusta GA, and Providence RI.\
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\cf0 www.climatecentral.org/news/across-u.s.-heaviest-downpours-on-the-rise-18989}